I wonder if your client would have had simular issues with a $6,000 leica with a $4000 lens.

I am not a pro - used to be, but that was before I enjoyed photography. If I were in your shoes I would simply point out to the client that I had showed them my work before they hired me and they liked it and if after I am done they are not happy with the product I
delivered then they are not obligated to accept it or pay for it.

when I did shoot for money my clients chose me based on my portfolio and references from prior clients - - - no one ever asked me what camera I used and when I showed up they didnt know the difference between the Hasselblad and the leica M2r that I was carrying

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NEX-7 & Sigma 30mm f/2.8
NEX-5 & 18-55 OSS
And a spare black 18-55

I did point out that out. Some people are just irrational I guess. It was enough to make me possibly shy about pulling it out at a shoot again.